1. Field of the Invention
The present invention relates to a recording medium determination apparatus configured to determine basis weight of a recording medium, and to an image forming apparatus.
2. Description of the Related Art
An image forming apparatus such as a copying machine or a laser printer using an electrophotographic process includes an image forming portion configured to form a toner image on an image bearing member, a transfer portion configured to transfer the toner image formed on the image bearing member onto a recording medium, and a fixing portion configured to heat and pressurize the recording medium to fix the toner image onto the recording medium. Hitherto, in such an image forming apparatus, a user sets a size or a type of the recording medium, for example, by an external apparatus such as a computer or via an operation panel installed in an image forming apparatus main body. Based on the setting, the image forming apparatus controls a transfer condition (e.g., transfer voltage or conveyance velocity of recording medium during transfer) and a fixing condition (e.g., fixing temperature or conveyance velocity of recording medium during fixing). In order to reduce such a user's setting burden, there has recently been offered an image forming apparatus that includes a sensor for determining a recording medium and thus automatically determines a type of the recording medium. In such an image forming apparatus, the type of the recording medium is automatically determined, and a transfer condition and a fixing condition are set in accordance with the determination result.
For example, as proposed in Japanese Patent Application Laid-Open No. S57-132055, there is known a sensor for determining basis weight (mass per unit area) of a recording medium by irradiating the recording medium with an ultrasonic wave and receiving the attenuated ultrasonic wave via the recording medium. When such a sensor using the ultrasonic wave (hereinafter referred to as ultrasonic sensor) is used for determining basis weight, it is desired to keep circumstances surrounding the sensor (e.g., atmospheric pressure and temperature) under certain conditions. This is because amplitude of the ultrasonic wave is known to change depending on surrounding circumstances thereof and affect the determination result of the basis weight. However, the circumstances in which the image forming apparatus including the ultrasonic sensor is installed are not always constant. As a method of reducing such influences, for example, in Japanese Patent Application Laid-Open No. 2010-18433, there is proposed a method of reducing or cancelling the influence of circumstance variation based on amplitude of an ultrasonic wave in an absent condition of any recording medium.
As the method of reducing the influence of circumstance variation on the basis weight determination, as described above, there is known a method of cancelling the influence of circumstance variation by measuring an ultrasonic output in the absent condition of any recording medium to perform correction. However, factors affecting the basis weight determination carried out by using the ultrasonic sensor are not limited to the circumstance variation. For example, even when a positional relationship (distance) between a transmission unit and a receiving unit of the ultrasonic wave changes, amplitude of the received ultrasonic wave changes. When the circumstance correction is carried out under this condition, the ultrasonic output in the absent condition of any recording medium also changes, and thus correction accuracy may reduce. Therefore, it is desired to install the transmission unit and the receiving unit of the ultrasonic wave in the image forming apparatus in such a manner that the positional relationship therebetween does not change. However, because of circumstances described below, an installing position of the basis weight detection sensor using the ultrasonic wave is limited, and it is difficult to maintain constant the positional relationship between the transmission unit and the receiving unit.
When the basis weight is determined by the ultrasonic wave, the transmission unit and the receiving unit of the ultrasonic wave need to be arranged so as to sandwich the recording medium, and it is accordingly difficult to integrate the transmission unit and the receiving unit. Further, because the transfer condition and the fixing condition are determined based on the determination result of the basis weight detection sensor, the sensor needs to be arranged on an upstream of a conveyance path of the recording medium, that is, on at least before the transfer portion configured to transfer the toner image formed on the image bearing member onto the recording medium. The image forming apparatus generally has a mechanism for removing the recording medium in case that the recording medium stays on the conveyance path (jamming). Accordingly, for example, the transfer unit is an open/close type in many cases, and one of the transmission unit and the receiving unit of the ultrasonic wave is arranged on the transfer unit. However, when the sensor is installed in such an openable/closable and movable portion, a position of the sensor may shift due to the open/close operation. Thus, there is a high risk that the positional relationship (distance) between the transmission unit and the receiving unit of the ultrasonic wave cannot be maintained constant.